Microbial reduction and in situ product crystallization coupled with biocatalyst cultivation during the synthesis of 6R-dihydrooxoisophorone

An in situ product crystallization procedure was developed for a crystalline product formed during microbial reduction coupled with cell cultivation. The model reaction was the asymmetric reduction of 4-oxoisophorone (OIP) by baker's yeast (Saccharomyces cerevisiae). Yeast cells were cultivated fed-batch to reach a maximum concentration of 30 gdw·L^-1. The desired product, 6R-dihydrooxoisophorone (DOIP), may be further reduced by baker's yeast to an unwanted by-product; thus, DOIP was removed immediately from the fermenter via an external crystallization loop in this procedure. The OIP reduction rate was five times higher (≅0.33 mmol·gdw ^-1·h^-1) as compared to the reduction rate with resting cells. OIP reduction was started when the optimum cell concentration had already been reached in the reactor because the substrate (OIP) at ≥55 mM concentration inhibited cell growth. An appropriate supply of glucose as carbon and energy source was necessary to support the coupled reactions involving cell growth and maintenance and product formation while avoiding formation of metabolic by-products. Final DOIP yield and selectivity were 85% and 99%, respectively, while over 100 g·L^-1 of product was obtained in the crystallizer. The product crystals with favorable properties were readily recovered from the crystallizer. These results indicate that product crystallization is not impaired by the solutes present in the fermentation medium.